The present invention relates to signal detection apparatus and in particular, to processing apparatus for enhancing the strength of the received signal.
When an acoustic or electromagnetic wave travels through an air or water medium, it is refracted and scattered due to randomly-varying inhomogenities within the medium itself. Thus, both the phase and amplitude of the wave vary at the receiver even though the source may have constant output power and may be fixed in the medium. Further, due to the different paths travelled in reaching the receiver and the different directions from which the energy is received, the differing phases and amplitudes have a pronounced tendency to interfere destructively.
In the past, the multi-path interference problem has been partially solved by using a receiver with a narrow beam pattern which, in principle, selects the direction of the maximum energy input (the `direct` ray) and minimizes responses from other directions (`secondary` rays). Frequency filters also have been widely used to improve the signal-to-noise ratio. In particular, with the advent of digital computers and the development of Fast Fourier Transforms (FFT) algorithms to determine the strength of a signal in the presence of noise, the filter band width has been narrowed to a limit which depends practically upon the breadth of the frequency variation of the fluctuating signal, as well as its amplitude.
As is known, the output of the FFT consists of a real and an imaginary component, or, in mathematical language: EQU P (pressure amplitude) = a + ib (1)
The output voltage of the signal then can be determined by the relationship: EQU V = k P 2+ b.sup.2).sup.1/2 ( 2)
Even so, it is to be noted that only the pressure amplitude, P, is detected and, that, in particular, this amplitude information is obtained only from one preferential direction. In other words, such techniques do not accept and make use of the excluded secondary rays provided by multi-path signals and, as is known, these secondary signals sometimes are as large as the direct signal rays.
It is therefore a primary object of the present invention to provide a system which utilizes the secondary rays to increase the strength of the signal and to decrease its fluctuation amplitude.
Another object is to utilize the previously-excluded phase information provided by the FFT to achieve the present purposes.
In general, these objects are achieved by providing separate channels for individually processing each of the multi-path waves or rays to provide amplitude and phase information for each frequency present in each wave and, subsequently, to adjust the relative phases of identical frequencies present in each channel to permit the amplitudes of all frequencies to be added constructively.